Electronic oven including microwave coupling structure and folded cavity filters therefor



3,511,958 UCTURE AND ELECTRONIC OVEN INCLUD UPLING STR FOLDED CAVITYFILTERS THEREFOR 5 Sheets-Sheet 1 Filed Dec. 51, 1968 Fla.

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May 12, 1970 J. E. STAATS 3,511,953 ELECTRONIC OVEN INCLUDING-MICROWAVECOUPLING STRUCTURE AND FOLDED CAVITY FILTERS THEREFOR I Filed Dec. 31,1968 Sheets-Sheet 5 j W W 64 67 62 65 L FIG. 9

5 a: k E STOP BAND Q 5 Ii k 4- E E 2 72 k 7/ O I I I I I I l l FREQUENCYIN. G/GAHERTZ United States Patent 3,511,958 ELECTRONIC OVEN INCLUDINGMICROWAVE COUPLING STRUCTURE AND FOLDED CAVITY FILTERS THEREFOR James E.Staats, Louisville, Ky., assignor to General Electric Company, acorporation of New York Filed Dec. 31, 1968, Ser. No. 788,204 Int. Cl.H05b 9/06; H03h 7/10 US. Cl. 219-1055 ABSTRACT OF THE DISCLOSURE Thereis disclosed an electronic oven comprising a generator of microwaveenergy of a predetermined frequency coupled to a cooking cavity and to asource of DC and 60-cycle AC operating potentials by a coaxialtransmission line, the inner conductor of the transmission lineincluding structure defining a folded resonant cavity rejection filterfor the second harmonic of the predetermined frequency formed by twocup-like conductors facing in opposite directions and disposed onewithin the other, and also including a capacitive bypass filter betweenthe inner and outer conductors for the second and higher harmonics ofthe predetermined frequency.

15 Claims This invention is concerned with an improved transmission lineforming a part of an electronic oven and, more particularly, withimproved microwave filtering elements for the transmission line.

It is a general object of this invention to provide an electronicheating apparatus including structure defining a cooking cavity, agenerator for generating electromagnetic wave energy and a transmissionline for coupling the generator to the cooking cavity, wherein thetransmission line includes improved resonant cavity filters forattenuatingthe second harmonic of the operating frequency of thegenerator, which filters comprise a compact structure occupying arelatively small portion of the transmission line.

It is a further object of this invention to provide in an electronicheating apparatus, structure defining a cooking cavity, a generator forgenerating electromagnetic wave energy of a predetermined ultra-highfrequency and having a pair of output terminals, a transmission linecoupling the generator to the cooking cavity and including a hollowouter conductor coupled to one terminal of the generator and an innerconductor coupled to the other terminal of the generator and a filtersection in the inner conductor and comprising structure defining aninner space and an outer space disposed about the inner space and apassage interconnecting the inner space and the outer space to provide afolded cavity, the filter section structure having a length equal toabout one-eighth of the wavelength of the second harmonic of thepredetermined frequency, the folded cavity having an effectiveelectrical length equal to one-quarter of the wavelength of the secondharmonic of the predetermined frequency and being resonant at the secondharmonic of the predetermined frequency, the filter section presenting ahigh impedance to the transmission of the second harmonic of thepredetermined frequency along the inner conductor while having a lengthequal to about one-eighth of the wave- 3,511,958 Patented May 12, 1970"ice length of the second harmonic of the predetermined frequency,whereby during the transmission of the electromagnetic wave energy alongthe transmission line, the second harmonic of the predeterminedfrequency is highly attenuated by the filter section while thepredetermined frequency is propagated to the cooking cavitysubstantially without attenuation.

In connection with the foregoing object it is another object of thisinvention to provide an electronic heating appartus of the type setforth, wherein the transmission line includes a first cup-like filtermember including a cylindrical side wall surrounding the adjacentportion of the inner conductor concentric therewith and spaced therefromand an end wall closing the side Wall at one end thereof and connectedto the inner conductor, the cylindrical side Wall having a length equalto about one-eighth of the wavelength of the second harmonic of thepredetermined frequency, and a second cup-like filter member including acylindrical side wall surrounding the cylindrical side wall of the firstfilter member concentric therewith and spaced therefrom and an end wallclosing the side wall of the second filter member at one end thereof andconnected to the inner conductor, the cylindrical side wall of thesecond filter member having a length equal to about one-eighth of thewavelength of the second harmonic of the predetermined frequency, thefirst and second cup-like filter members cooperating with each other andwith the inner conductor to form a folded cavity having an effectiveelectrical length equal to one-quarter of the Wavelength of the secondharmonic of the predetermined frequency and being resonant at the secondharmonic of the predetermined frequency.

Yet another object of this invention is to provide an electronic heatingapparatus of the type set forth, wherein the transmission line includesa filter section in the inner conductor and comprising structuredefining first and second inner spaces and first and second outer spacesrespectively disposed about the first and second inner spaces and firstand second passages respectively interconnecting the first inner andouter spaces and the second inner and outer spaces to provide first andsecond folded cavities, the filter section having a length equal toabout one-quarter of the Wavelength of the second harmonic of thepredetermined frequency, the first and second folded cavities eachhaving an effective electrical length equal to one-quarter of thewavelength of the second harmonic of the predetermined frequency andeach being resonant at the second harmonic of the predeterminedfrequency, the filter section thereby providing two resonant cavitieseach presenting a high impedance to the transmission of the secondharmonic of the predetermined frequency along the inner conductor.

In connection with the foregoing object, a further object of thisinvention is to provide an electronic heating apparatus of the type setforth wherein each of the first and second folded cavities is defined byfirst and second cup-like filter members concentrically disposed onewithin the other along the inner conductor and so arranged that the endwalls of the inner ones of the cup-like members abut one another.

It is another object of this invention to provide a transmission linefor interconnecting a generator for generating electromagnetic Waveenergy of a predetermined ultra-high frequency and a load wherein thegenerator has a pair of output terminals, the transmission linecomprising a hollow outer conductor coupled to one terminal of thegenerator, an inner conductor disposed within the outer conductorconcentric therewith and coupled to the other terminal of the generator,and a filter section in the inner conductor and comprising structuredefining an inner space and an outer space disposed about the innerspace and a passage interconnecting the inner space and the outer spaceto provide a folded cavity, the filter section structure having a lengthequal to about one-eighth of the wavelength of the second harmonic ofthe predetermined frequency, the folded cavity having an effectiveelectrical length equal to onequarter of the wavelength of the secondharmonic of the predetermined frequency and being resonant at the secondharmonic of the predetermined frequency, the filter section presenting ahigh impedance to the transmission of the second harmonic of thepredetermined frequency along the inner conductor while having a lengthequal to about one-eighth of the wavelength of the second harmonic ofthe predetermined frequency, whereby during the transmission ofelectromagnetic wave energy along the transmission line the secondharmonic of the predetermined frequency is highly attenuated by thefilter section while the predetermined frequency is propagated to theload substantially without attenuation.

In connection with the foregoing object, it is another object of thisinvention to provide a transmission line of the type set forthcomprising a first cup-like filter member including a cylindrical sidewall surrounding the adjacent portion of the inner conductor concentrictherewith and spaced therefrom and an end wall closing the side Wall atone end thereof and connected to the inner conductor, the cylindricalside wall having a length equal to about one-eighth of the wavelength ofthe second harmonic of the predetermined frequency, and a secondcup-like filter member including a cylindrical side wall surrounding thecylindrical side Wall of the first filter member concentric therewithand spaced therefrom and an end wall closing the side wall of the secondfilter member at one end thereof and connected to the inner conductor,the cylindrical side wall of the second filter member having a lengthequal to about one-eighth of the wavelength of the second harmonic ofthe predetermined frequency, the first and second cup-like filtermembers cooperating with each other and with the inner conductor to forma folded cavity having an effective electrical length equal toone-quarter of the wavelength of the second harmonic of thepredetermined frequency and being resonant at the second harmonic of thepredetermined frequency.

In connection with the foregoing objects, yet another object of thisinvention is to provide a transmission line of the type set forthcomprising structure defining first and second inner spaces and firstand second outer spaces respectively disposed about the first and secondinner spaces and first and second passages respectively interconnectingthe first inner and outer spaces and the second inner and outer spacesto provide first and second folded cavities, the filter sectionstructure having a length equal to about one-quarter of the wavelengthof the second harmonic of the predetermined frequency, the first andsecond folded cavities each having an effective electrical length equalto one-quarter of the wavelength of the second harmonic of thepredetermined frequency and each being resonant at the second harmonicof the predetermined frequency, the filter section thereby providing tworesonant cavities each presenting a high impedance to the transmissionof the second harmonic of the predetermined frequency along the innerconductor.

In connection with the foregoing object, another object of the inventionis to provide a transmission line of the type set forth wherein each ofthe first and second folded cavities is defined by first and secondcup-like filter members concentrically disposed one within the otheralong the inner conductor and so arranged that the end walls of theinner ones of the cup-like filter members abut one another.

Further features of the electronic heating apparatus pertain to theparticular arrangement of the parts of the transmission line andcoupling assembly forming a part thereof, whereby the above-outlined andadditional operating features thereof are attained.

The invention both as to its organization and its method of operation,together with objects and advantages thereof will best be understood byreference to the following specification taken in connection with theaccompanying drawings, in which:

FIG. 1 is a side elevational view with certain portions broken away ofan electronic heating apparatus made in accordance with and embodyingthe principles of the present invention;

FIG. 2 is a fragmentary view on an enlarged scale in vertical section ofa crossed-field discharge device forming a part of the generator for theapparatus of FIG. 1;

FIG. 3 is a fragmentary view in vertical section through thetransmission line forming a part of the apparatus of FIG. 1 andinterconnecting the device of FIG. 2 to the cooking cavity of theapparatus and including a harmonic filter according to a firstembodiment of the present invention;

FIG. 4 is an enlarged fragmentary view of that portion of thetransmission line of FIG. 3 including the harmonic filter according to afirst embodiment of the invention;

FIG. 5 is a view in vertical section along the line 5-5 of FIG. 4;

FIG. 6 is a fragmentary view in vertical section through thetransmission line forming a part of the apparatus of FIG. 1 andincluding a harmonic filter according to a second embodiment of theinvention;

FIG. 7 is an enlarged fragmentary view of that portion of thetransmission line of FIG. 6 including the harmonic filter according to asecond embodiment of the present invention;

FIG. 8 is a schematic representation of the equivalent electricalcircuit for the harmonic filter of FIG. 7; and

FIG. 9 is a chart showing the frequency response of the filter of FIG.4.

Referring now to FIG. 1 of the drawings, the electronicheating apparatus10 there illustrated, and embodying the features of the presentinvention, is in the form of a combination electric and electronic rangethat is especially designed for home use. More particularly, the range10 comprises an upstanding substantially boxlike casing 11 formed ofsteel and including a pair of side walls 12, a rear wall 13 having aremovable closure member or panel 15 disposed therein, and a top wall-14 and a bottom wall 16, the removable panel 15 being removably held inplace by a plurality of screws 27. The casing 11 houses in the uppercentral portion thereof a metal liner 20 defining a heating or cookingcavity 21 therein, the metal liner 20 preferably being formed of steel,and essentially comprising a box-like structure provided with a top wall22, a bottom wall 23, a rear wall 24, and a pair of opposed side walls25; whereby the liner 20 is provided with an upstanding front openinginto the heating cavity 21 defined therein. Further, the casing 11 isprovided with a front door 28 arranged in a front opening formed thereinand cooperating with the front opening provided in the liner 20, thefront door 28 being mounted adjacent to the lower end thereof uponassociated hinge structure 29; whereby the front door 28 is movablebetween a substantially vertical closed position and a substantiallyhorizontal open position with respect to the front opening provided inthe liner 20.

An electric heating unit 36 is arranged in the upper portion of theheating cavity 21 adjacent to and below the top wall 22, and an electricheating unit 37 is arranged in the lower portion of the heating cavity21 adjacent to and above the bottom wall 23; which electric heatingunits 36 and 37 are utilized in the carrying out of conventional bakingand broiling cooking operations in the heating cavity 21, as explainedmore fully hereinafter. Also, a temperature sensing bulb 38 is arrangedin a pocket provided in one of the side walls 25; which temperaturesensing bulb 38 forms a part of an oven switch and temperaturecontroller and is utilized in carrying out the previously mentionedconventional baking and broilingoperations in the heating cavity 21. Theside walls 25 of the liner 20 further carry thereon a plurality of shelfsupports 26 for the supporting of shelves (not shown) that in turnsupport items to be cooked within the heating cavity 21, There also isprovided below the front door 28 a lower front panel 19 that closes afront opening in the lower portion of the casing 11, the bottom wall 16further being provided with a reticulated member or screen 17 and thelower portion of the rear wall 13 being provided with a reticulatedmember or screen 18, the screens 17 and 18 permitting the passage of airthrough the lower portion of the casing 11 to cool the electronicapparatus therein as will be described more fully hereinafter.

Disposed below the liner 20 and spaced therefrom is a generallyhorizontally arranged lower baffle 30 extending laterally across thecasing 11 between the side walls '12 thereof and extending rearwardlyfrom the front of the casing 11 to a point spaced forwardly of the rearWall 13, the baffle 30 in cooperation with the casing 11 defining abottom machinery compartment in the lower portion of the casing 11, thelower front panel 19 being removable to provide access to the bottommachinery compartment 35.

Mounted on the underside of the baflle 30 is an electric motor 31 havingan output drive shaft 32 including a reduced portion 33 extendingupwardly into the liner 20 and supporting thereon a turntable 34 formedof expanded sheet metal and arranged in the lower portion of the heatingcavity 21. The turntable 34 is mounted for rotation and upon operationof the motor 31 is adapted to support food to be heated or cooked in theelectronic operation that is carried out in the heating cavity 21, asexplained more fully hereinafter. The motor 31 has an associated geartrain (not shown) that reduces the speed of the shaft 33 toapproximately 6 rpm.

A rear baffle is provided to the rear of the liner 20 and extends acrossthe casing -11 between the side walls 12 thereof, the baffie 40including a main wall 41 carrying centrally thereof a rearwardly offsetwall 42 that is in general horizontal alignment with the rear of theliner 20 and spaced rearwardly therefrom. Disposed in the lower portionof the main wall 41 is an opening around which is disposed a flange 43connecting with and air duct 44 that communicates with the screen 18 inthe rear wall 13 of the casing 11. It further will be noted that thebottom bafiie 30 carries on the rear thereof an upwardly and rearwardlyextending baflle section 39 that extends toward the offset wall 42 butis spaced therefrom, the baffles 30 and 40 being formed of metal, suchas steel, whereby the spacing between the baflie section 39 and the rearbafile 40 minimizes the conduction of heat therebetween during theoperation of the range 10. The rear baffie 40 cooperates with the casing11 to provide a rear machinery compartment 45, the rear machinerycompartment 45 being disposed behind the liner 20 and access theretobeing provided through the removable panel 15 that covers the opening inthe rear wall 13 described above.

There is arranged to the right (as viewed in FIG. 1) of the bottommachinery compartment 35 a generator for supplying ultra-high frequencyelectromagnetic wave energy for electronic cooking within the cookingcavity 21, the generator 50 including a crossed-field electronicdischarge device 100 of the construction and arrangement disclosed inthe copending application of James E. Staats, Ser. No. 559,267, filedJune 21, 1966 now Pat. No. 3,458,755. Referring to FIGS. 1 and 2, itwill be seen that the device is disposed within a box-like structure orcasing 101 that extends completely about the device 100 but is open ontwo opposed sides thereof, the sides disposed to the left and right inFIG. 2, the device 100 and the associated parts therefor being mountedwithin and electrically connected to the casing 101. As will beexplained more fully hereinafter, high operating DC potentials arepresent on the casing 101, whereby it is desirable electrically toisolate and shield the casing 101, and to this end a second box-likestructure 102 has been provided that surrounds the casing 101 and isalso provided on an opposed pair of sides thereof with openings inalignment with the opposed open sides of the casing 101, all toaccommodate the passage of air through the casing 101 and the box-likestructure 103 to cool the device 100 and the associated parts housedtherein. However, in order to prevent contact of the user with thecasing 101 and the device 100 disposed therein, reticulated metal covers103 are provided for covering the openings in the opposed sides of theboxlike structure 102, the covers 103 permitting passage of airtherethrough and thus through the casing 101 and the structure 102 whilepreventing contact of a user with the casing 101 and the parts disposedtherein that carry the high operating DC potentials. Further, the casing101 is mounted upon insulators 104 carried by the structure 102, therebyelectrically to insulate the casing 101 from the structure 102.

As viewed in FIG. 1, the structure 102 and the casing 101 disposedtherein are disposed to the rear of the bottom apparatus 35, and aredisposed to the right within the bottom apparatus compartment 35 whenthe range 10 is viewed from the front. In order to provide cooling airfor passage through the reticulated covers 103 and across the device100, there has been provided an open ended housing 105 disposed to theleft in FIG. 1 or in front of the structure 102 and housing therein atleast a part of a voltage doubler and rectifier circuit that supplies DCoperating potentials to the device 100 and also houses therein a fan 106powered by a motor 55 within the housing 105, an air duct 107 beingprovided about the fan 106 to direct air therefrom into the structure102 and into the casing 101 and about the device 100. More specifically,the fan 106 operates to draw air through the screen 17 at the bottom ofthe apparatus compartment 35, the air being formed into a stream by thehousing 105 and passed over the rectifier 130, the motor 55 and throughthe air duct 107 into the structure 102; the air stream within thestructure 102 passes into the casing 101 and about the device 100 andcooling fins 118 disposed thereon and passes therefrom and into the airduct 44 to be discharged through the screen 18 in the rear wall 13 ofthe casing 11.

In accordance with the present invention, the air stream generated bythe fan 106 is used to cool all of the various parts of the generator50, and specifically the crossed-field discharge device 100 and thevoltage doubler and rectifier circuit 130 associated therewith. To thisend the housing 105 and the air duct 107 and the air duct 44 have beenprovided so as to concentrate the air stream upon the parts noted, allwhile attempting to deflect the air stream away from the baffies 30 and40. The baffles 30 and 40 further protect the liner 20 and the cookingcavity 21 disposed therein from the air stream thus generated so as tomaintain more uniform cooking conditions within the cooking cavity 21and thus to improve the cooking therein.

Referring now particularly to FIG. 2 the generator 50 there illustratedwill be described in greater detail. An electron discharge device 100 iscontained within a substantially cylindrical metal envelope 111 andincludes anode and cathode structure (not shown). Surrounding theenvelope 111 and connected thereto is a plurality of cooling fins 118for dissipating heat from the device 100 as explained above. In order toestablish a unidirectional magnetic field within the device 100 there isprovided a composite magnetic field winding 120a and 1201; disposed atthe upper and lower ends of the device 100 and connected in seriesrelation by a conductor 123. A DC operating potential from a voltagedoubler and rectifier 130 is applied to the winding 12% by a conductor122, and from the winding 120a to the device 100 by a conductor 124which is connected to one of the cooling fins 118 as at 125. Furtherdetails of the construction and operation of the device 100 aredisclosed in the previously mentioned Staats application Ser. No.559,267 now Pat. No. 3,458,755, the disclosure of which is incorporatedherein by reference.

The generator 50 is arranged to be advantageously operated in connectionwith suitable control and power supply apparatus, details of theconstruction and operation of which are disclosed in the copending US.applications of James E. Staats, Ser. No. 656,977, filed June 12, 1967now Pat. No. 3,421,115, Ser. No. 569,006, filed June 27, 1966 nowabandoned and Ser. No. 181,144, filed Mar. 20, 1962, the disclosures ofall of which applications are incorporated herein by reference.

The device 100 is operative to supply microwave energy at an ultra-highfrequency of about 915 mHZ., with a power output at the output terminalsin the general range of 50 to 800 watts. The device is arranged tosupply the RF power for cooking and to this end a lower transmissionline 300 extends from the device 100 and is coupled to an uppertransmission line 200 which extends to the cooking cavity 21, thetransmission lines 200 and 300 being of the coaxial type including aninner conductor and an enclosing outer conductor electrically insulatedtherefrom. Both of the output terminals of the device 100 are at asubstantial voltage DC with respect to ground potential, so that theoutput terminals are electrically insulated from ground potential, aswell as from each other. One of the output terminals is coupled by acapacitor 317 to the adjacent end of the outer output conductor, and theother output terminal is coupled by a capacitor 318 to the adjacent endof the inner output conductor, the remote end of the inner conductorprojecting as an antenna 201 into the oven cavity 21; the remote end ofthe outer conductor is electrically connected to the metal liner 20.Thus the RF power produced by operation of the device 100 is radiatedfrom the antenna 201 into the cooking cavity 21, so as to producecooking effects upon food arranged therein, all in a conventionalmanner.

An important feature of the present invention resides in theconstruction and connection of the transmission lines 200 and 300interconnecting the crossed-field discharge device 100 and the liner 20,a detailed description of the transmission lines 200 and 300 being nowpresented with particular reference to FIGS. 3 through 5 of thedrawings. The crossed-field discharge device 100 is provided with amagnet yoke 126 at the upper end thereof which connects to the anode ofthe device 100 (not shown) and forms an outer conductor and an outputterminal for the device 100, the lower end of the magnetic yoke 126being more particularly connected to the anode of the device 100 and theupper end extending upwardly through the field winding 12012 and beingconnected to a magnet flange 127. The lower end of the device 100 islikewise provided with a magnet yoke 128 having the upper end thereofconnected to the anode of the device 100 and the other end extendingdownwardly through the field winding 120a and being connected at thelower end to a magnet flange 129. The cathode of the device 100 (notshown) has connected thereto a stud 381 forming a part of the uppercoupling structure 380, the stud 381 and the magnet yoke 128 forming acoaxial output connection for the device 100.

Referring specifically to FIG, 3, it will be seen that 8 the output fromthe other end of the upper transmission line 200 is coupled to theantenna 201 disposed in the heating cavity 21, the antenna 201 beingessentially a rod having a threaded stud 202 at one end thereof, theantenna 201 extending into the cavity 21 a short distance below the topwall 22 and essentially midway between the side walls 25, see FIG. 1.The stud 202 extends into a complementarily threaded opening at the endof a rod 217 forming an inner conductor that extends into the cavity 21,the conductor 217 being held in position by a bracket 204 and acooperating electrical insulator 208. More specifically, the bracket 204includes an annular mounting flange 205 which is suitably secured as bywelding to the rear wall 24 of the liner 20 and disposed about anopening therein, the flange 205 in turn carrying a forwardly directedannular flange 206 terminating in an inturned holding flange 207disposed agaist the forward surface of the insulator 208.

A top section 210 of the upper transmission line 200 is formed by afirst tube 211 forming the outer conductor therefor and the innerconductor 217. The forward edge of the tube 211 has an outwardlydirected flange 212 thereon which is suitably secured as by welding onthe rear side of the rear wall 24 and surrounding the opening thereinand assisting in holding the insulator 208 in the operative position incooperation with the bracket 204.

The rear end of the tube 211 is received in a T 220, the T 220 includinga body 221 and a pair of arms 222 and 124 and a leg or arm 226 allprovided with a seat or shoulder 223, 225 and 227, respectivelyinternally thereof. The rear end of the tube 211 is disposed within thearm 222 and in telescopic relationship therewith and has the rearmostend thereof spaced slightly forwardly with respect to the shoulder 223,an insulator 218 being disposed between the tube 211 and the shoulderand held in position thereby, the insulator having a central openingtherein receiving the inner conductor 217 therethrough. Preferably theouter end of the arm 222 has longitudinally extending slots (not shown)formed therein and receives therearound a clamp 215 having associatedtherewith a screw 216 (see FIG. 1) for tightening the clamp 215 aboutthe outer end of the arm 222, thus releasably to clamp the arm 222 aboutthe rear end of the tube 211 to hold it in the operative position. Itfurther is pointed out that the arm 222 extends through an opening 46 inthe batfle wall 42, whereby the major portion of the T 220 is disposedto the rear of the baflle wall 42 and thus in the rear machinerycompartment 45, whereby the T 220 is accessible for service andmaintenance through the removable panel 15 (see FIG. 1 also The otherarm 224 carries on the outer end thereof a dust cover 228 that serves toclose the same to prevent the entry of dust, dirt, water and the likeinto the interior of the T 220.

A rear section 230 for the upper transmission line 200 is provided by anouter tube 231 and an inner conductor 232 disposed within the outer tube231. The upper end of the tube 231 extends into the lower arm or leg 226of the T 220 and is spaced from the shoulder 227 thereof, an insulator.234 being disposed between the upper end of the tube 231 and theshoulder 227 and held in position therebetween. The insulator 234 has anopening therethrough to receive the upper end of the inner conductor232, the inner conductor 232 having an upper flattened end 233-having anopening therein receiving therethrough a screw 219 engaging in acomplementarily threaded opening in the rear end of the rod 217, wherebyto interconnect the rear end of the inner conductor 217 and the upperend of the inner conductor 232. There further is provided around thelower portion of the leg 226 one of the clamps 215, the lower end of theleg 226 preferably being slotted .(not shown) whereby tightening of thescrew 216 associated with the clamp 216 serves to tighten the leg 226about the upper end of the tube 231 releasably to hold the parts in theassembled telescoping positions.

The lower end of the rear transmission line section 230 is received in aT 240, the T 240 including a body 241 having a pair of arms 242 and 244and a leg on arm 246 each provided with a cooperating internal seat orshoulder 243, 245 and 247, respectively. More particularly, the leg 246is positioned upwardly and receives telescopically therein the lower endof the tube 231, the lower end of the tube 231 being spaced upwardlywith respect to the seat 247 to receive therebetween an insulator 236 tomount the same Within the T 240, the insulator 236 having an openingtherethrough receiving the lower end of the inner conductor 232therethrough. The outer end of the leg 246 is preferably slotted (notshown) and receives therearound one of the clamps 215, whereby by meansof the associated screw 216, the clamp 215 can be tightened about theouter end of the leg 246 to clamp it against the lower end of the tube231 releasably to hold the parts in the assembled position. The arm 244preferably carries thereon a dust cover 248 which cover serves toprevent the entry of dust, dirt, water and other debris into theinterior of the T 240 and the upper transmission lines 200 and 300.

There is operatively associated with the other arm 242 of the T 240 arear section 310 of the lower transmission line 300, the rear section310 including a tube 311 as the outer conductor and a tube 314 as theinner conductor spaced therefrom and electrically insulated therefrom.The tube 311 extends into and is telescopically associated with the arm242 of the T 240, the inner end of the tube 311 being spaced from theshoulder 243 and receiving therebetween an insulator 238 that is held inposition thereby. One of the clamps 215 is disposed about the outer endof the arm 242 which is preferably slotted (not shown), whereby by meansof the associated screw 216 the clamp 215 can be tightened to press thearm 242 about the adjacent end of the tube 311 releasably to hold theparts in the assembled position. The insulator 238 has an openingtherein receiving therethrough the inner conductor 314, the rear end ofthe inner conductor 314 carrying a plug 315 secured thereto and having athreaded opening therein, the lower end of the rear inner conductor 232having a flattened portion as at 235 having an opening therein receivinga screw 237 that threadedly engages th opening in the plug 315, thus tointerconnect the lower end of the inner conductor 232 and the rear endof the inner conductor 314. The forward end (the end disposed to theleft of FIG. 3) of the inner conductor 314 is outwardly flared as at 316to permit ready assembly with other portions of the lower transmissionline 300 as will be described more fully hereinafter. Telescopicallyoverlapping the tube 311, in surrounding relationship therewith, is anannular coupling member 313, having an enlarged diameter portion 319extending to the left of the tube 311 as viewed in FIG. 3.Telescopically received within the enlarged portion 319 of the couplingmember 313 is a tube 312 having a diameter slightly greater than thediameter of the tube 311.

The rear lower section 310 cooperates with and is telescopicallyassociated with a front lower transmission line section 320, the frontlower section 320 including a tube 321 serving as the outer conductorand an annular inner conductor 322 disposed within the tube 321 andspaced therefrom and electrically insulated therefrom. The tube 321 hasan external diameter slightly less than the internal diameter of thetube 312 whereby the rear end of the tube 321 is telescopically receivedtherein, a layer of insulation 317 being disposed therebetween toprovide a capacitive coupling at RF frequencies. The annular innerconductor 322 has received in an opening 323 therethrough and threadedlyengaged therewith a probe or inner conductor 324, the probe 324 havingan external diameter slightly less than the internal diameter of theinner conductor 314, whereby the rear end of the probe 324 can bedisposed therein, a layer of insulation 31 8 being provided therebetweenand forming therewith a capacitive coupling at RF frequencies.

The rearward end of the inner conductor 322 has a hollow portiongenerally designated 325 and including a relatively thin cylindricalside wall 327 and an annular end wall 328. The cylindrical side wall 327is disposed in surrounding relationship with the adjacent portion of theinner conductor 324 concentric therewith and spaced therefrom. Disposedwithin the hollow end portion 325 of the inner conductor 322 is acup-like electrically conductive filter member 330 having a cylindricalside wall 331 closed at the rearward end thereof by an annular end wall332. The cylindrical side wall 331 is disposed in surroundingrelationship with the adjacent portion of the inner conductor 324concentric therewith and spaced therefrom. The side wall 331 is alsospaced slightly from the side wall 327 of the inner conductor 322, andthe open end of the side wall 331 is spaced longitudinally rearwardlyfrom the end wall 328 of the inner conductor 322. The end wall 332 ofthe cup-like member 330 has an internally threaded opening 333 therein,the inner conductor 324 being received therethrough and threadedlyengaged thcreat with the cup-like member 330. The inner conductor 324,the hollow end 325 of the inner conductor 322 and the cup-like member330 all cooperate to define a folded cavity 340 comprising an annularinner space 337 between the inner conductor 324 and the side wall 331,an outer annular space 338 defined between the side walls 327 and 331and a passage 339 interconnecting the inner and outer spaces 337 and 338at the forward ends thereof. An annular insulator 335 is disposed aboutthe inner conductor 324 and abuts the outer surface of the end wall 332and the open end of the cylindrical side wall 327, thereby closing thefolded cavity 340 and insulating the side walls 327 and 331 from eachother. A lock nut 336 is threadedly engaged with the inner conductor 324and bears against the insulating spacer 335 fixedly to retain the partsin their assembled position. The folded cavity 340 is of such dimensionsas to be resonant at the second harmonic of the operating frequency ofthe device to provide a rejection filter for the second harmonic as willbe more fully described hereinafter.

The forward end of the front lower transmission line section 320 iscoupled to the crossed-field discharge device 100 by means of a T 370,the T 370 including a body 371 having a pair of arms 372 and 374 and aleg or arm 376 each provided with a cooperating internal seat orshoulder 373, 375 and 377, respectively. More particularly, the leg 376is positioned downwardly and is telescopically received in the upper endof the magnet yoke 126 for coupling thereto, the T 370 forming a part ofthe upper coupling structure 380. Also forming a part of the uppercoupling structure 380 is a bullet 382 receiving into the lower endthereof the stud 381 that is coupled to the cathode of the device 100,the bullet 382 having a shoulder 383 thereon that cooperates with theshoulder 377 to hold in operative position an insulator 378 that servesto center the bullet 382 with respect to the leg 376. The upper end ofthe bullet 382 abuts the forward end of the inner conductor 322 andcarries an internally threaded eye 384 through which is threadedlyinserted the probe 324, the rear end of the probe 324 being threadedlyreceived in the complementarily threaded opening 323 in the innerconductor 322, thereby to support the forward end of the inner conductor322, there also being provided therearound a supporting insulator 367.The forward end of the tube 321 is telescopically received within theleg 372 and is spaced a short distance away from the shoulder 373,thereby to clamp the insulator 367 in the operative positiontherebetween. Preferably the forward end of the tube 321 is fixedlysecured as by soldering to the arm 372.

A decoupling structure is carried by the other arm 374 of the T 370, andmore particularly comprises an inner conductor 385 having a rear endconnected to the forward end of the probe 324 by means of a threadedconnection therebetween, the inner conductor 385 having a reducedforward portion 386. Fixedly mounted on the arm 374 and telescopicallyreceived therein is an outer conductor 387 within which is disposed aninsulating sleeve 379. Disposed within the insulating sleeve 379 is acup-like inner conductor 388 having the outer end closed by a wall 389,a screw passing through an opening in the outer wall 389 and engaging ina threaded opening in the forward end of the reduced portion 386 of theinner conductor 385 fixedly to interconnect the parts and to provide theinput terminal 368.

It is noted that the outer conductors 312 and 321 overlap a distancecorresponding to about one-quarter wavelength at the freqeuncy ofoperation of the device 100 and the inner conductor 314 overlaps theprobe 324 a distance corresponding to about one-quarter of thewavelength of the frequency of operation of the device 100, thereby toprovide a filter for the second and higher even harmonics of thefrequency of operation of the device 100. However, in the operation ofmicrowave generators of the type set forth herein for use withelectronic cooking apparatus some harmonics of the frequency ofoperation of generator, particularly the second, may be of sufficientpower level to require a more effective filter than that provided by theusual choke and capacitive filters. For example, the above-describedcapacitive filter has a fairly narrow band width on the order of aboutmHz. which is insufficient to cover the wide frequency pulling rangecaused by various food load impedances in an electronic oven.Accordingly, there is provided by this invention a fairly broad bandrejection filter for the second harmonic of the frequency of operationof the device 100, the method of operation of which willnow be morefully described. The cylindrical side wall 327 of the hollow end portion325 of the inner conductor 322 has a length equal to about one-eighth ofthe wavelength of the second harmonic of the frequency of operation ofthe device 100. However, the side wall 327, in cooperation with the sidewall 331 and the inner conductor 324 and the end wall 328 defines acavity having an effective electrical length equal to one-quarter of thewavelength of the second harmonic, the cavity being formed by the outerspace 338, the connecting passage 339 and the inner space 337. There isthereby defined a folded cavity 340 of such dimensions as to form aquarter-wave resonant cavity at the second harmonic of the frequency ofoperation of the device 100, which resonant cavity presents a highseries impedance of the tarnsmission of this second harmonic along thetransmission line 300 thereby producing high attenuation of this secondharmonic frequency. The above-described filter structure is particularlyadvantageous in that it provides a quarter-wave resonant cavity in asection of transmission line having a length equal to only aboutoneeighth of the wavelength of the frequency of resonance, therebyaffording a valuable economy of space. Furthermore, the folded-cavityharmonic filter of this invention has been found to provide low costconstruction, low insertion loss, high power capability, a high degreeof harmonic attenuation and fairly broad band operation.

In FIG. 9, there is shown a curve 70 representing the frequency responseof the folded-cavity harmonic filter of this invention, the powertransmitted through the filter in microwatts being plotted along theordinate and the frequency in gigahertz being plotted along theabscissa. The response curve 70 has a relatively flat portion 75 of highattenuation. From FIG. 9 it can be seen that the stop band of the filtermeasured from the point 71 to the point 72 has a width of about 40 mHz.,the filter providing at least 40 db attenuation over this stop band. Itis noted that the filter has fairly sharp cutoff at the low frequencyend of the stop band, it being understood, of course, that only thebottom of the response curve is shown, the curve 70 extending nearlyvertically upward to approximately 800 watts. Thus, while the filterprovides very high attenuation of the second harmonic, the fundamental(915 mHz.) frequency of operation of 12 the device is passed by thefilter substantially without attenuation.

In the transmission line 300, the bullet 382, the T 370, the innerconductor 322 and the tube 321 are shaped and arranged to provide aquarter-wave transformer section at the frequency of operation of thedevice 100- More particularly, the shouldered portion of the bullet 382and the conductor 322 have dimensions such that the impedance of thedevice 100 is matched to the impedance of the transmission lines 200 and300 that are in turn matched to the impedance of the heating cavity 21.Likewise, the bullet 382, the inner conductor 385, the outer conductor387 and the inner conductor 388 are all shaped and arranged to provide aquarter wave transformer section that assists in decoupling RF energyfrom the input terminal 368 to prevent the propagation of RF energy intothe power supply. Furthermore, the inner conductor 385, the outerconductor 387, the insulator 379, the inner conductor 388 and the endwall 389 cooperate to provide the parallel capacitive impedance andreactive impedance. To this end the distance between the inner surfaceof the end wall 389 and the center of the bullet 382 preferablycorresponds to about one-quarter wavelength of the frequency ofoperation of the device 100, and the conductors 387 and 388 overlap adistance corresponding to about one-eighth wavelength of the frequencyof operation of the device 100. It is noted that the steppedconfiguration of the inner conductor 385 permits a shorter mechanicalconnection while maintaining an electrical characteristic equavilent toone-quarter wavelength of the operating frequency of the device 100.

Referring now to FIGS. 6 and 7, there is shown a second embodiment ofthe folded-cavity harmonic filter according to this inventoin wherebyeven greater attenuation of the second harmonic of the frequency ofoperation of the device 100 is attained by providing two folded cavityfilters connected in series along the inner conductor of thetransmission line 300. The structure of FIG. 6 is essentially the sameas the structure of FIG. 3 with the exception of the inner conductorportion of the forward section 320 of the lower transmission line 300.Therefore, the description of that portion of the structure of FIG. 6which is identical to the structure of FIG. 3 will not be here repeatedand the identical portions of FIGS. 3 and 6 are shown with the samereference numerals in both figures.

The rear lower section 310 of the transmission line 300 cooperates withand is telescopically associated with the front lower section 320, asdescribed above, the front lower section 320 including the tube 321serving as the outer conductor and the probe 324 serving as the innerconductor. There is provided along the inner conductor 324 a filtersection 420 comprising a pair of rejection filters for the secondharmonic of the frequency of operation of the device 100. Disposedadjacent to the forward end of the inner conductor 324 is a cup-likeelectrically conductive filter member 425 comprising a cylindrical sidewall 426 surrounding the adjacent portion of the inner conductor 324concentric therewith and spaced therefrom and being closed at theforward end thereof by an end Wall 427, the outer surface of which abutsthe eye portion of the stud 382. An inner filter member 430 is disposedrearwardly from the end wall 427 about the inner conductor 324. Theinner filter member 430 is cylindrical in shape comprising a solidcentral portion 431 and a pair of hollow end portions 432 and 438, theend portion 432 and 438 respectively comprising cylindrical side walls433 and 439 closed at the inner ends thereof by the central portion 431.The cylindrical side wall 433 is disposed within the cylindrical sidewall 426 of the cup-like member 425 and is concentric with and disposedapproximately midway between the side wall 426 and the inner conductor324. Surrounding the other cylindrical end 438 of the inner filtermember 430 is a second cup-like filter member 435 comprising acylindrical side wall 436 surrounding the side wall 439 of the innerfilter member 430 concentric therewith, the side wall 436 being closedat the rearward end thereof by an end wall 437. The side wall 439 of theinner filter member 431 is surrounded by the cup-like member 435 and isdisposed approximately midway between the side wall 436 and the innerconductor 324 concentric therewith. Disposed around the central portion431 of the inner filter member 430 is an annular insulating spacer 421insulating the open ends of the cylindrical side walls 426 and 436 fromeach other and from the inner filter member 430. An annular insulatingspacer 422 is disposed about the inner conductor 324 and abuts the outersurface of the end wall 437 of the cup-like member 435. The parts of thefilter section 420 are all held in their assembled position by a locknut 423 threadedly engaged with the inner conductor 324 and bearingagainst the insulating spacer 422.

It can be seen from FIGS. 6 and 7 that the filter section 420 isessentially a combination of two folded-cavity filters of the type shownin FIG. 3 arranged back-to-back. The cup-like filter members 425 and 435each have a length corresponding to about one-eighth of the wavelengthof the second harmonic of the frequency of operation of the device 100.The inner conductor 324, the inner filter member 430 and the cup-likefilter members 425 and 435 cooperate with one another to define twocavities 440, each having an effective electrical length equal toone-quarter of the wavelength of the second harmonic frequency, and eachcomprising an inner annular space 428, an outer annular space 429 and aconnecting passage 434 interconnecting the spaces 428 and 429 at one endthereof. There are thereby defined two folded cavities 440, each havingsuch dimensions as to be resonant at the second harmonic of thefrequency of o eration of the device 100, and each thereby presenting ahigh series impedance to the transmission along the transmission line300 of the second harmonic to effect high attenuation thereof. Each ofthe folded cavities in the filter section 420 provides all of theadvantages afforded by the filter of FIG. 3, but the filter section 420achieves a compact and economical series arrangement of two such filtersections to afford increased attenuation of the second harmonic.

There is shown in FIG. 8 the schematic representation of alumped-impedance equivalent circuit, generally designated 60, for thefilter section 420 of FIG. 6. The circuit includes a pair of conductors61 and 68, respectively corresponding to the inner and outer conductorsof the transmission line 300, the conductor 68 being grounded as at N.Connected in series relationship in the conductor 61 are two parallelresonant circuits 62 and 65, each comprising an inductive impedance anda capacitive impedance. More particularly, the parallel resonant circuit62 comprises a capacitance 63 and an inductance 64 while the parallelresonant circuit 65 comprises a capacitance 66 and an inductance 67.Each of the parallel circuits 62 and 65 is tuned to resonance at thesecond harmonic of the frequency of operation of the device 100 and theyrespectively correspond to the two folded resonant cavities of filtersection 420. The resonant circuits 62 and 65 thus present a high seriesimpedance to transmission of the second harmonic along the conductor 61while transmitting the fundamental frequency of operation of the device100 substantially without attenuation. Connected in parallel with theresonant circuits 62 and 65 is a capacitance 69 corresponding to theinsulating spacer 421 in FIG. 6 and having a value such as to present alow impedance to the frequency of operation of the device 100 whilepresenting a high impedance to the second harmonic thereof.

Because the potentials for operating the device 100 are derived from thevoltage doubler and rectifier circuit 130, neither the stud 381 formingthe inner conductor or any of the parts such as the yoke 126 and the T370 forming the outer conductor of the coupling structure 380 can begrounded. However, it is highly desirable to ground the portion of thetransmission line 200 disposed to the rear of the rear baffle 40, and tothis end the capacitive coupling at 317 and 318 has been providedbetween the rear lower transmission line portion 310 and the front lowertransmission line portion 320. Furthermore, the outer conductors 312 and321 overlap a dis tance corresponding to about one-quarter wavelength atthe frequency of operation of the device and the inner conductor 314overlaps the probe 324 a distance corresponding to about one-quarterwavelength of the frequency of operation of the device 100, whereby toprovide a filter for the second and higher even harmonics as well asproviding a DC insulation between the parts named. Accordingly, theouter conductor 312 can be grounded as on the casing 11 and the bafilemember 40, thereby to present only grounded parts to workmen gainingaccess to the rear machinery compartment 45 through the removable panel15.

It further is pointed out that the T 220, the entire rear transmissionline section 230, the T 240 and the rear lower transmission line section310 form a removable transmission line assembly that can be bodily movedrearwardly through the opening provided by the removable panel 15 formaintenance and repair of the parts. Such movement of the transmissionline assembly rearwardly is accomplished by simply loosening the clamp215 about the leg 222 which frees the T 220 from the tube 211 andremoving the screw 219 to free the inner conductor 232 from the innerconductor 217. Due to the telescoping arrangement of the lowertransmission line portions there is no need to remove or disconnect anyparts other than by the relative sliding movement of the conductors 314and 324 with respect to each other and of the coupling member 313 andtube 312 with respect to each other. Reassembly of the parts isfacilitated by the flared end 316 on the inner conductor 314 and by theenlarged diameter portion 319 on the coupling member 313. It further isnecessary to hold the removable transmission line assembly in theassembled position, and to this end a spring 249 under tension has beenprovided interconnecting the T 240 and the baffie wall 42, thuscontinually to urge the removable transmission line assembly into theassembled operative position. It is also noted that the front panel 19of the range 10 is preferably removable to afford easy access to thedevice 100 which, together with the front portion 320 of thetransmission line 300, may be disengaged from the rear portion 310 ofthe line and removed through the front of the machinery compartment 35for servicing.

Another important feature of the transmission lines 200 and 300 residesin the fact that the tubes 211, 231, 312, 321 and 387 can all be formedessentially of standard tubing shaped as required and cut to length, thetubing preferably being formed of copper, brass or other goodelectrically conductive metal. The inner conductors 232 and 314 also canbe formed essentially of standard tubing shaped as required and cut tolength, this tubing also preferably being formed of copper, brass or thelike. The Ts 220, 240 and 370 are also of standard configuration and areall identical one to the other, the Ts preferably being formed ofcopper, brass or other material having good electrical conductivity.Finally, the insulators 218, 234, 236, 238, 335, 367, 378, 421 and 422are preferably all formed of a polytetrafiuoroethylene resin such asthat sold under the trademarkTeflon.

A lower coupling structure 390 cooperates with the magnet yoke 128serving as an outer conductor of a transmission line and a tube 391connected at the upper end to one end of the heater of the device 100(not shown) and serving as an inner conductor of a coaxial transmissionline, the lower end of the tube 391 carrying an insert 392 therein andreceiving the screw 396 serving as an input terminal. Disposed withinand essentially lining the magnet yoke 128 is a sleve 393 ofelectrically insulating material, an inner conductor 394 being disposedagainst the sleeve 393 and telescopically overlapping a portion of theyoke 1'28 and having the outer end thereof closed by an end wall 395,the end wall 395 having an opening therethrough receiving the shank ofthe screw 396 that engages in a complementarily threaded opening in theinsert 392. The yoke 128, the tube 391, the insulating sleeve 393, theinner conductor 394 and the end wall 395 cooperate to provide a parallelresonant circuit including a reactive impedance and a capacitiveimpedance, the structure comprising a high impedance to RF energy toprevent propagation thereof onto the conductor 155. More specifically,the distance between the lower adjacent end of the anode of the device100 and the inner surface of the end wall 395 is equivalent to a quarterwavelength at the operating frequency of the device 100, and the yoke128 and the inner conductor 394 telescopically overlap a distanceequivalent to one-eighth wavelength at the operating frequency of thedevice 100.

From the above it will be seen that the liner is effectively isolatedfrom the bottom machinery compartment 35 and the rear machinerycompartment 45 by the baffle members 30 and 40, respectively, thereby toprovide a more uniform distribution of heat within the liner 20 and thusto permit good cooking therein. The entire generator 50 including thecrossed-field discharge device 100 and the voltage doubler and rectifiercircuit 130 therefor are housed within the bottom machinery compartment35 which provides a protecting housing therefor. The fan 106 serves tocool all of the electrical components of the generator 50 by drawing airinwardly through the screen 17 into the bottom machinery compartment 35and across the crossed-field discharge device 100 and outwardly throughthe screen 18. The stream of air thus created is effectively preventedfrom coming into contact with the liner 20 due to the presence of thebaffle members 30 and 40. Also, the improved coupler structure andtransmission line 200 has been provided, the major portion of which canbe readily removed from the assembled relation with the liner 20 and thedevice 100 for repair and services purposes through the removable panel15, and can thereafter be readily reassembled therewith.

Finally, there has been provided a, folded-cavity rejection filter forthe second harmonic of the frequency of operation for the device 100,which filter affords economical and compact construction allowing theconstruction of a quarter-wave resonant cavity in a section oftransmission line having a length equal to only about one-eighth of thewavelength of the frequency of resonance. There has also been provided aharmonic filter which affords broad-band operation, low insertion loss,high power capability and a high degree of harmonic attenuation. It willbe understood, of course, that while the filters described above aredesigned to filter the second harmonic of the frequency of operation ofthe device 100, they may be designed to filter any desired harmonic withappropriate changes in the dimensions of the various parts.

From the above it will be seen that there has been provided an improvedelectronic heating apparatus 10 and microwave coupling structure 380390and improved transmission lines 200 and 300 therefore and improvedharmonic filters in the transmission line 300 which fulfill all of theobjects and advantages set forth above.

While there has been described what is at present considered to be thepreferred embodiments of the invention, it will be understood thatvarious modifications can be made therein, and it is intended to coverin the appended claims all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:

1. In an electronic heating apparatus, structure defining a cookingcavity, a generator for generating electromagnetic wave energy of apredetermined ultra-high frequency and having a pair of outputterminals, a transmission line coupling said generator to said cookingcavity and including a hollow outer conductor coupled to one terminal ofsaid generator and an inner conductor coupled to the other terminal ofsaid generator, and a filter section in said inner conductor andcomprising structure defining an inner space and an outer space disposedabout said inner space and a passage interconnecting said inner spaceand said outer space to provide a folded cavity, said filter sectionstructure having a length equal to about one-eighth of the wavelength ofthe second harmonic of the predeten mined frequency, said folded. cavityhaving an effective electrical length equal to one-quarter of thewavelength of the second harmonic of the predetermined frequency andbeing resonant at the second harmonic of the predetermined frequency,said filter section presenting a high impedance to the transmission ofthe second harmonic of the predetermined frequency along said innerconductor while having a length equal to about one-eighth of the wavelength of the second harmonic of the predetermined frequency, wherebyduring the transmission of electromagnetic wave energy along saidtransmission line the second harmonic of the predetermined frequency ishighly attenuated by said filter section while the predeterminedfrequency is propagated to said cooking cavity substantially withoutattenuation.

'2. The electronic heating apparatus set forth in claim 1, and furtherincluding a second filter section in said transmission line including ahigh impedance path in said inner and outer conductors for the secondand higher even harmonics of the predetermined frequency to effect highattenuation thereof.

3. In an electronic heating apparatus, structure defining a cookingcavity, a generator for generating electromagnetic wave energy of apredetermined ultra-high frequency and having a pair of outputterminals, a transmission line coupling said generator to said cookingcavity and including a hollow outer conductor coupled to one terminal ofsaid generator and an inner conductor coupled to the other terminal ofsaid generator, a first cup-like filter member including a cylindricalside wall surrounding the adjacent portion of said inner conductorconcentric therewith and spaced therefrom and an end wall closing saidside wall at one end thereof and connected to said inner conductor, saidcylindrical side wall having a length equal to about one-eighth of thewavelength of the second harmonic of the predetermined frequency, and asecond cup-like filter member including a cylindrical side wallsurrounding the cylindrical side wall of said first filter memberconcentric therewith and spaced therefrom and an end wall closing saidside wall of said second filter member having a length equal to nectedto said inner conductor, said cylindrical side wall of said secondfilter members having a length equal to about one-eighth of thewavelength of the second harmonic of the predetermined frequency, saidfirst and second cup-like filter members cooperating with each other andwith said inner conductor to form a folded cavity having an effectiveelectrical length equal to one-quarter of the wavelength of the secondharmonic of the predetermined frequency and being resonant at the secondharmonic of the predetermined frequency, said first filter member beingdisposed within said second filter member and said second filter memberhaving a length equal to about one-eighth of the wavelength of thesecond harmonic of the predetermined frequency while presenting a highimpedance to the transmission of the second harmonic of thepredetermined frequency along said inner conductor, whereby during thetransmission of electromagnetic Wave energy along said transmission linethe second harmonic of the predetermined frequency is highly attenuatedby said filter members while the predetermined frequency 4. Theelectronic heating apparatus set forth in claim 3, and further includinga filter section in said transmission line including a high impedancepath in said inner and outer conductors for the second and higher evenharmonic of the predetermined frequency to effect high attenuationthereof.

5. The electronic heating apparatus set forth in claim 3, wherein eachof said inner and outer conductors comprises two portions insulated fromeach other and telescopically overlapping a distance equal to aboutonequarter of the wavelength of the predetermined frequency to provide alow impedance capacitive bypass path between said inner and outerconductors for the second and higher harmonics of the predeterminedfrequency.

6. The electronic heating apparatus set forth in claim 3, wherein theend walls of said first and second cup-like filter members arerespectively disposed in opposite directions along said inner conductorand wherein said second cup-like filter member surrounds and is spacedfrom said first cup-like filter member.

7. The electronic heating apparatus set forth in claim 3, wherein theend walls of each of said first and second cup-like filter members hasan internally threaded opening therein, said inner conductor beingdisposed through said internally threaded openings and being threadedlyengaged thereat with said first and second cup-like filter members.

8. In an electronic heating apparatus, structure defining a cookingcavity, a generator for generating electromagnetic wave energy of apredetermined ultra-high frequency and having a pair of outputterminals, a transmission line coupling said generator to said cookingcavity and including a hollow outer conductor coupled to one terminal ofsaid generator and an inner conductor coupled to the other terminal ofsaid generator, and a filter section in said inner conductor andcomprising structure defining first and second inner spaces and firstand second outer spaces respectively disposed about said first andsecond inner spaces and first and second passage respectivelyinterconnecting said first inner and other spaces and said second innerand outer spaces to provide first and second folded cavities, saidfilter section structure having a length equal to about one-quarter ofthe wavelength of the second harmonic of the predetermined frequency,said first and second folded cavities each having an effectiveelectrical length equal to one-quarter of-the Wavelength of the secondharmonic of the predetermined frequency and each being resonant at thesecond harmonic of the predetermined frequency, said filter sectionhaving a length equal to about one-quarter of the Wavelength of thesecond harmonic of the predetermined frequency while providing tworesonant cavities, each presenting a high impedance to the transmissionof the second harmonic of the predetermined frequency along said innerconductor, whereby during the transmission of electromagnetic waveenergy along said transmission line the second harmonic of thepredetermined frequency is highly attenuated by said filter sectionwhile the predetermined frequency is propagated to said cooking cavitysubstantially without attenuation.

9. The electronic heating apparatus set forth in claim 8, and furtherincluding a second filter section in said transmission line including ahigh impedance path in said inner and outer conductors for the secondand higher even harmonics of the predetermined frequency to effect highattenuation thereof.

10. In an electronic heating apparatus, structure defining a cookingcavity, a generator for generating electromagnetic wave energy of apredetermined ultra-high frequency and having a pair of outputterminals, 2. transmission line coupling said generator to said cookingcavity and including a hollow outer conductor coupled to one terminal ofsaid generator and an inner conductor coupled to the other terminal ofsaid generator, and a first cup-like filter member including acylindrical side wall surrounding the adjacent portion of said innerconductor concentric therewith and spaced therefrom and an end wallclosing said side wall at one end thereof and connected to said innerconductor, said cylindrical side wall having a length equal to aboutone-eighth of the wavelength of the second harmonic of the predeterminedfrequency, and a second cup-like filter member including a cylindricalside wall surrounding the cylindrical side wall of said first filtermember concentric therewith and spaced therefrom and an end wall closingsaid side wall of said second filter member at one end thereof andconnected to said inner conductor, said cylindrical side wall of saidsecond filter member having a length equal to about one-eighth of thewavelength of the second harmonic of the predetermined frequency, saidfirst and second cup-like filter members cooperating with each other andwith said inner conductor to form a first folded cavity having aneffective electrical length equal to onequarter of the wavelength of thesecond harmonic of the predetermined frequency and being resonant at thesecond harmonic of the predetermined frequency to effect highattenuation thereof, a third cup-like filter member including acylindrical side wall surrounding the adjacent portion of said innerconductor concentric therewith and spaced therefrom and an end wallclosing said side wall of said third filter member at one end thereofand abutting the end wall of said first filter member and connected tosaid inner conductor, said cylindrical side of said third filter memberhaving a length equal to about one-eighth of the wavelength of thesecond harmonic of the predetermined frequency, and a fourthcup-like'filter member including a cylindrical side wall surrounding thecylindrical side wall of said third filter member concentric therewithand spaced therefrom and an end wall closing said side wall of saidfourth filter member at one end thereof and connected to said innerconductor, said cylindrical side wall of said fourth filter memberhaving a length equal to about one-eighth of the wavelength of thesecond harmonic of the predetermined frequency, said third and fourthcup-like filter members cooperating with each other and with said innerconductor to form a second folded cavity having an effective electricallength equal to one-quarter of the wavelength of the second harmonic ofthe predetermined frequency and being resonant at the second harmonic ofthe predetermined frequency to effect high attenuation thereof, wherebyduring the transmission of electromagnetic wave energy along saidtransmission line the second harmonic of the predetermined frequency ishighly attenuated by said filter members while the predeterminedfrequency is propagated to said cooking cavity substantially withoutattenuation.

11. The electronic heating apparatus set forth in claim 10, and furtherincluding a filter section in said transmission line including a highimpedance path in said inner and outer conductors for the second andhigher even harmonics of the predetermined frequency to effect highattenuation thereof.

12. The electronic heating apparatus set forth in claim 10, wherein eachof said inner and outer conductors comprises two portions insulated fromeach other and telescopically overlapping a distance equal to aboutonequarter of the wavelength of the predetermined frequency to provide ahigh impedance path in said inner and outer conductors for the secondand higher even harmonics of the predetermined frequency.

13. The electronic heating apparatus set forth in claim 10, wherein theend walls of said first and second cuplike filter members arerespectively disposed in opposite directions along said inner conductorand the end walls of said third and fourth cup-like filter members arerespectively disposed in opposite directions along said inner conductor,and wherein said first and third cup-like filter members arerespectively spaced from said second and fourth cup-like filter members.

14. The electronic heating apparatus set forth in claim 19 2o 13,wherein the end walls of said first and third cup-like 2,877,433 3/1959Devot. filter members are integral 'With each other. 3,187,277 6/1965Wantuch 15. The electronic heating apparatus set forth in claim l 14,and further including an annular insulating spacer 3439144 4/1969 et a1219-1055 disposed about the end Walls of said first and third cup- 53,411,114 11/1968 Schmld 3 like filter members and separating thecylindrical side Walls of said second and fourth cup-like filter membersJOSEPH Primary EXaminer from each other' References Cited L. H. BENDER,Assistant Examiner UNITED STATES PATENTS 10 Us. Cl. 2,700,136 1/1955Devot. 33373 2,851,666 9/1958 K'aich.

